Methylidene malonates are compounds having the general formula (I):
wherein R1 and R2 may be the same or different and represent H or a C1 to C18 hydrocarbon group or heterohydrocarbon group having one or more nitrogen, halogen, or oxygen atoms; provided that R1 and R2 are not both H. Such compounds have been known for well over half a century and their value in both organic synthesis and polymer chemistry is well known. Similarly, the use of these compounds as or as a component of adhesives, including skin bonding adhesive; molding materials; and the like is equally well known. Yet, despite all the promise, these compounds have found limited commercial success owing to the difficulty of their production; the poor, though improving, yet still erratic, yields; and the general instability of these compounds.
Numerous processes have been developed for the production of methylidene malonates having a formula similar to or falling within the formula of formula (I) above. Two of the earliest methods for the production of methylene dialkyl malonates, the simplest of the methylidene malonates, were the iodide method in which methylene iodide was reacted with dialkyl malonates and the formaldehyde method in which formaldehyde was reacted with dialkyl malonates in the presence of a base, in solution in alcohol solvents. The former was unsatisfactory due to very low yield and expensive starting materials. The latter, though periodically giving better yields than the iodide process, gave relatively poor yields and, more critically, was widely inconsistent from batch to batch, even under the same conditions.
Despite this inconsistency, early efforts continued to focus on the formaldehyde method. One of the most widely practiced formaldehyde methods consisted of reacting diethyl malonate with formaldehyde in glacial acetic acid in the presence of a metal acetate catalyst to produce the diethyl methylidene malonate. The latter was subsequently recovered by distillation following removal of the catalyst by filtration and separating off the solvent. These efforts continued to frustrate and various modifications and iterations of this basic process were developed all in an effort to improve the consistency and yields associated therewith.
Bachman et. al. (U.S. Pat. No. 2,313,501) taught the reaction of a C1-C5 dialkyl malonate with formaldehyde in the presence of an alkali metal salt of a carboxylic acid, in solution in a substantially anhydrous carboxylic acid solvent, followed by fractional distillation to separate the desired product. Bachman et. al. indicate that their process is advantageously carried out in the presence of inhibitors of the polymerization of monomeric vinyl compounds. Suitable inhibitors are said to include the copper salts such as copper chloride and, especially, copper salts of carboxylic acids such as cupric acetate, iron salts such as ferric acetate, and phenols, such as hydroquinone. These are added to the solution mix before the addition of the malonate.
Although Bachman et. al. reported yields of up to 72%, the results presented are conversion rates, not yields. Looking at the actual yields of the process, Bachman et. al.'s best performance was a yield of 43% with all others being less than 25%. Though Bachman et. al. speak of high purity and the ability to recover pure material, they never present any details or data as to what those purities or recoveries were. In any event, Bachman et al. reported that the isolated product, upon standing, polymerized in a day to several weeks time depending upon the purity of the isolated material, which polymer was then heated to facilitate the reversion of the polymer to the monomer.
D'Alelio (U.S. Pat. No. 2,330,033), on the other hand, alleged that such processes were erratic and more often produced yields that averaged 10 to 12 percent. D'Alelio espoused an improved process with yields on the order of 30% and higher by reacting a malonic acid ester with formaldehyde in a ratio of one mole of the former to at least one mole of the latter under alkaline conditions and, in most cases, in the presence of a polymerization inhibitor such as copper, copper acetate, hydroquinone, resorcinol, or catechol, to form a methylol derivative. The methylol derivative is then acidified to a pH below 7.0 using a suitable organic or inorganic acid in order to retard further reaction. The acidified mass is then dehydrated to form the corresponding methylidene malonate which is subsequently separated by distillation.
Coover et. al. (U.S. Pat. No. 3,221,745 and U.S. Pat. No. 3,523,097) took another approach to the formation of the methylidene malonates, electing to begin with a preformed dialkyl alkoxymethylenemalonate. In accordance with their process, the olefinic double bond of the latter compound was subjected to hydrogenation in the presence of a hydrogenation catalyst and the hydrogenated compound was then subject to pyrolysis in the presence of a phosphorous pentoxide inhibitor to strip off the alcohol to produce the methylene malonate. The resultant mass was then subjected to vacuum distillation at low temperature to separate an allegedly high purity methylidene malonate, though with a low yield. According to Coover et. al., the use of low temperature distillation is said to prevent the contamination of the monomer with pyrolytic products that commonly result from high temperature distillation. These high purity monomers are said to be especially important for surgical applications.
In discussing the critical need for high purity materials, Coover et. al. draw particular attention to the extreme sensitivity of their monomers to the presence of even small amounts of acidic and basic impurities, the former inhibiting polymerization leading to sluggish and ineffective adhesive activity and the latter accelerating polymerization leading to unstable and useless products. They indicate that the amount of such impurities should not exceed 100 ppm, preferably not more than 10 ppm. Unfortunately, other than discussing its limitations with respect to the acidic and basic impurities, and despite its contention of high purity materials, Coover et. al. never provide any data pertaining to the purity of their materials. Clearly, though, these materials are not “pure” materials inasmuch as Coover et. al., like the others before them and since, require redistiliation of the “pure” distillate.
Additionally, although suggesting that their high purity materials “have reasonably good” stability when stored in bulk, they recommend the addition of low levels, 0.0001 to 0.01 weight percent, of a polymerization inhibitor to the monomer materials in order to increase storage stability. Suitable polymerization inhibitors are said to include sulfur dioxide, hydroquinone, nitric oxide, organic acids, boron trifluoride, hydrogen fluoride, stannic chloride, ferric chloride, and organic anhydrides. To assist with cure, particularly cure speed, Coover et. al. also suggest the addition of cure accelerators or catalysts to their formulated adhesives, but cautions against adding them too early as they would cause premature polymerization.
Despite the erratic nature of the aforementioned processes, there were continued efforts to find improved processes for the production of methylidene malonates with a focus on more consistent and reliable processes with improved yields and higher purity. These effort focused not only on the simple methylidene malonates of the early art but also on finding new routes that allowed for the formation of a broader array of methylidene malonates, including symmetrical and asymmetrical species as well as those whose ester functionality was more complex, e.g., having a higher carbon number, unsaturation, heteroatoms and the like.
Eventually, such efforts led to multi-step processes in which certain unsaturated molecules served as a platform for the formation of intermediate adducts from which the methylidene malonates were subsequently stripped and recovered. For example, Hawkins et. al. (U.S. Pat. No. 4,049,698) found that certain malonic diesters could be reacted with formaldehyde and a linear, conjugated diene in the presence of a primary, secondary or tertiary amine at about reflux temperature to form an intermediate adduct that could then be readily pyrolyzed at temperatures in excess of 600° C. to split off the desired methylidene malonate. Similarly, Ponticello (U.S. Pat. No. 4,056,543) and Ponticello et. al. (U.S. Pat. No. 4,160,864) developed processes by which asymmetrical methylene malonates, especially methyl allyl methylene malonate, were prepared from previously formed norbornene adducts, the latter having been prepared by the Diels-Alder reaction of an alkyl acrylate with cyclopentadiene at room temperature or with heating or use of a Lewis catalyst. The so formed monoester norbornene adducts were then reacted with an electrophile material in the presence of an alkyl-substituted lithium amide complex to form the diester adduct and subsequently pyrolyzed at a temperature of 400° C. to 800° C. at a pressure of 1 mm to 760 mm Hg in an inert atmosphere to strip off the desired methylene malonates. These efforts, despite their gains in yield and/or purity, still failed to achieve commercial success.
Citing numerous disadvantages of the foregoing processes, which disadvantages were said to make them difficult, if not impossible, to adapt to industrial scale, Bru-Magniez et. al. (U.S. Pat. No. 4,932,584 and U.S. Pat. No. 5,142,098) developed a process whereby anthracene adducts were prepared by reacting mono- or di-malonic acid ester with formaldehyde in the presence of anthracene, most preferably in a non-aqueous solvent medium in the presence of select catalysts. According to Bru-Magniez et. al., the anthracene adducts were said to be readily produced in high yields with the desired methylidene malonates obtained by stripping them from the anthracene adduct by any of the known methods including heat treatment, thermolysis, pyrolysis or hydrolysis; preferably heat treatment in the presence of maleic anhydride. The resultant crude products were then subjected to multiple distillations, preferably lower temperature distillations under vacuum, to recover the purified methylidene malonate. Despite the claim to high yields, their crude yields were generally in the range of 21-71%, more importantly, nothing is said with respect to the purity of the material obtained.
Based on conversations with the successors to the Bru-Magniez technology, efforts to commercially produce the material have met with great difficulty owing to the high instability of the overall production process and final products. Indeed, they reported a high failure rate: of the limited batches that actually survived through crude distillation, the resultant products had to be stored in a freezer even after stabilizing with upwards of 50,000 ppm SO2 due to their high instability and spontaneous polymerization. Indeed, our own attempts to follow the prior art processes, including the Bru-Magniez process, most often resulted in failure owing to sublimation of the paraformaldehyde, a failure to produce the desired product (as evidenced by a lack of double bonds in the reaction product), and, more frequently, polymerization of the reaction mix and/or the crude yield. Even when a successful run was realized, it has now been found that the purity of the materials was quite low. Though the traditional analytical tests employed, including, the boiling point, fraction temperature, and refractive index suggests good yield and purity, further, more sophisticated analysis has found that these reaction products actually contained a number of analogs of the desired methylidene malonate, in addition to the desired material, as well as various byproducts. For example, in our efforts to produce 1-ethoxycarbonyl-1-ethoxycarbonyl methylene oxycarbonyl ethane (the 2.1.2 monomer), we found that besides the 2.1.2 monomer, the reaction products, even after initial separation and distillation, contained substantial amounts of the di-substituted and unsubstituted analogs (the 2.1.1.2 and 2.2 analogs, respectively) and oligomers and polymers of the foregoing, as well as various byproducts, especially glutarates. Consequently, though yields were presumably higher than achieved by other methods, purity was not as high as hoped and, as found through subsequent effort, repeatability was erratic at best.
While these advances in the art promoted somewhat higher yields and greater versatility, particularly with respect to the broader variety methylidene malonates, lingering problems persisted, namely batch-to-batch inconsistency, if not outright failure, and the general instability of the subsequent isolation and purification efforts and, for those products that survived, the instability of the so-formed products, especially in bulk storage, and of formulated products, such as adhesives, made with the same.
Due to the inherent problems with instability of the isolation and purification processes, focus instead seemed to focus on efforts to stabilize whatever products were recovered as well as formulated products containing those recovered monomers. For example, Ponticello and Ponticello et. al. suggested that the resulting products could be better stabilized by the addition of certain acidic polymerization inhibitors such as sulfur dioxide, hydrogen fluoride, boron trifluoride, nitric oxide, organic acids, organic anhydrides, stannic chloride and ferric chloride or certain free radical inhibitors such as hydroquinone, catechol, and monomethyl ether of hydroquinone. Although the aforementioned Bru-Magniez et. al. patents did not discuss the inclusion of polymerization inhibitors in their isolated monomer, a review of their subsequent patents demonstrating the utilization of the so formed methylidene malonates made clear that they too employed SO2 as a polymerization inhibitor of the formed methylidene malonates: a fact subsequently confirm in personal conversations with the successors to the Bru-Magniez technology. For example, Bru-Magniez et. al. (U.S. Pat. No. 6,640,461; U.S. Pat. No. 6,610,078; and U.S. Pat. No. 6,750,298) all speak of the need to degas the monomer under vacuum to remove the polymerization inhibitor SO2. Malofsky et. al. (U.S. Pat. No. 6,512,023) theorized that the stability of 1,1-disubstituted ethylene monomer and polymers could be improved by the use of specific combinations of certain vapor phase and certain liquid phase anionic polymerization inhibitors. While methylidene malonates fall within that broad class of materials. Malofsky et. al. only demonstrated and, in the prosecution of their patent, argued specificity and uniqueness of their solution to cyanoacrylate monomers and monomer compositions, distinguishing over efforts to stabilize the production of the monomers as well as other monomers.
While Bru-Magniez et. al. certainly achieved many benefits and made significant advances in the production of methylidene malonates and while the addition of the high levels of SO2 polymerization inhibitor to the isolated methylidene malonates and products containing them led to improved bulk storage stability and overall formulated product stability, freezer storage was still required, or strongly recommended, and Bru-Magniez' enthusiasm and accolades relative to industrial scale production were soon found to be tempered by continued inconsistency and instability in production as well as yields that, while higher, were still commercially undesirable, if not unviable. For example, Regula et. al., (U.S. Pat. No. 5,550,172), seemingly in endeavoring to follow the teachings of Bru-Magniez et. al., were only able to attain yields of less than 60 percent based on the adduct, though of high purity. Similarly, our own efforts to duplicate the results attained by Bru-Magniez et. al., even on a bench scale, resulted in wide variation in yields with very few attempts achieving or even coming close to those recited in Bru-Magniez. Indeed, on many occasions our efforts failed altogether due to the in-situ polymerization of the reaction mix in the reactor vessel.
Consequently, despite all the efforts and advances made in the art and the apparent desirability for these materials, no one has yet been able to adequately address the underlying and critical problems of process instability and inconsistency in the production of the methylidene malonates. It is this erratic nature of the production process and the attendant costs associated therewith that compromises and overshadows the commercial value and opportunity for these products.
Thus, if the methylidene malonates are ever to realize their commercial potential and promise, particularly in applications other than niche, high value added applications whose pricing can better offset the losses, costs and low yields of current processes, improved processes must be developed, especially processes that provide for more consistent and predictable yields. However, it has also been found that yield alone is not sufficient. Indeed, it has now been found that purity of the monomer, purity that goes far beyond the concerns with acidic and basic impurities as forewarned by Coover et. al., plays an important role in the cure or polymerizable characteristics of these materials and, perhaps most importantly, the properties of the polymerized materials. This is especially true for adhesive type applications for these materials.
Thus, there is a need for processes for the production of methylidene malonates that are not fraught with process failures, widely varying yields, unstable products, and unintended polymerizations and other by-products.
Furthermore, there remains a need in the industry for improved processes for the production of methylidene malonates wherein the formation of byproducts, such as glutarates, and dimers, oligomers and polymers of the methylidene malonates as well as thermal degradation products of the foregoing and the starting reactants, are lessened, if not avoided, particularly during the separation and fractionation steps for the recovery of the methylidene malonates.
In particular there is a need for a process that consistently achieves crude yields in excess of 35%, preferably in excess of 45%, more preferably in excess of 50%, especially with purities of the desired product and its analogs on the order of 80%, preferably 90% or more. Indeed, it would be phenomenal to attain purified yields on the order of 30% or more, let alone 40% or more, wherein the resultant product contained less than 8%, preferably less than 6%, most preferably less than 4%, of impurities and less than 12%, preferably less than 10%, most preferably less than 8% of the analogs of the desired product, on a consistent basis, and most preferably without the use of an intermediate adduct. Furthermore, such products would realize their true capabilities if one could produce the same on a commercial scale at a cost comparable to that for the production of cyanoacrylate monomer, in terms of actual costs, yields and/or purity.
Similarly, and in following therewith, there is a ongoing need for methylidene malonates whose bulk and long term storage stabilization is attained without concern for, or certainly less concern with respect to, the impact of such stabilization on the subsequent polymerization characteristics of the so formed methylidene malonates and which can be stored at room temperature. In particular, there remains a need and desire for methylidene malonates that do not require low temperature storage and/or further processing, such as degassing or the addition of scavengers, to remove stabilizers and polymerization inhibitors before the methylidene malonates can be formulated into end-use products and/or used in their intended end-use applications.
Finally, there is a need and desire for methylidene malonates that do not require, or require less, catalyst, polymerization activator and/or accelerator and the like, than heretofore required, in order to attain a sufficient degree and/or speed of polymerization, especially in adhesive and like bonding applications.